During recent years, the number of wet scrubber systems installed to remove gaseous and particulate material from waste stack gases has increased tremendously. Additional such systems are on the drawing board now. These wet scrubbers are being used to clear effluent from boiler stacks, incinerator stacks, lime kilns, foundries, blast furnaces, basic oxygen furnaces (BOF), open hearth units, coke plants, paper mill recovery boilers, pet food manufacturing facilities, electric furnaces (steel and aluminum), smelters, asphalt plants and many others.
One of the most important features of a scrubber system is the contact chamber, the device used to effect transfer of gaseous and/or particulate matter from the gas to the water phase. Most wet scrubber systems involve a venturi, a packed bed, an orifice plate, a spray chamber or a turbulent bed. Some systems even use two contact chambers in series, for example, a venturi followed by a spray chamber.
Venturi or orifice plate scrubbers are generally more efficient for particulate removal while packed beds, turbulent beds and spray chambers are usually more efficient for removing gaseous components such as SO.sub.2 and HF.
The present invention is directed specifically to those scrubber systems where scaling and deposition problems due to insoluble calcium carbonate, calcium fluoride, iron oxide (Fe.sub.2 O.sub.3), silica, manganese oxide, iron ore fines and slag fines are encountered. The type problem which the present invention minimizes is that which is found in gas scrubber systems of blast furnace operations where iron ore is being converted or processed to iron having a high carbon content.
In order to assure a complete understanding of the problem to which the present invention is addressed, a brief description of a blast furnace operation is hereafter provided.
In the production of iron, iron ore is fed together with additional ingredients such as dolomite through the top of a blast furnace fired by coke. An air stream is blown upward from the bottom of the furnace through the subsequent molten materials. The carbon of the coke reduces the iron ore (Fe.sub.2 O.sub.3) to iron metal. The molten iron is tapped from the bottom of the furnace while the slag is tapped from the middle of the furnace. The by-product of the carbon reduction is of course a combination of carbon dioxide and carbon monoxide which reacts with the calcium present to form the troublesome scaleformer, calcium carbonate and other solids: clay, slag, fines, etc.
As can be appreciated, the air stream blown upward contributes significantly to the impurity content of the flue gas, thus putting an extreme burden on the scrubbing system. The particulate load in the scrubbing medium ranges from about 1,000 to 2,000 parts per million because of the particulate load of the flue gas.
The scrubbers that are used in blast furnace gas clean-up are often of the Venturi design and treat the off-gases from the furnace. These gases contain significant quantities of iron oxide, whose fine particle size allows it to be carried off in the gas stream. Also present may be coke fines, to a lesser extent and to some extent, particulate slag materials used, such as silicates and unused dolomites. The iron oxide has been subjected to high temperatures within the furnace and may therefore be in a sintered form of low surface activity. However, its fine particle size presents deposition problems in scrubbers and delivery lines.
Another example of an operative system is a basic oxygen furnace (BOF) in which the BOF receives molten metal from the blast furnace plus scrap, various alloys to meet specifications, and lime and fluorspar as a flux. Oxygen is introduced through a lance to remove impurities. The oxygen blow can release 4.4 tons of dust per 220 ton heat. This dust must be removed from the waste gas. The dust consists of iron oxide, lime and fluoride.
This particulate and soluble gases are removed from the waste gas in a wet scrubber. In the scrubber water the iron oxide, calcium fluoride and calcium carbonate combine to cause massive deposit build-up on the scrubber system intervals which result in inefficient scrubber operation and high maintenance costs.
Prior art methods, such as the method set forth in U.S. Pat. No. 3,880,620, employ scale inhibitors, i.e., inorganic and organic phosphates; and low molecular weight polymeric dispersants. This approach has not been completely successful in preventing deposition and has resulted in increased treatment costs and frequent shutdowns for mechanical cleaning.
Accordingly, it is an object of this invention to provide a composition and method of effectively preventing deposits in gas scrubbers.
It is a further object of this invention to provide a composition and method for preventing deposits in gas scrubbers which is economical and which minimizes the frequency of mechanically cleaning scrubber equipment.
It is still a further object of this invention to provide a composition and method of preventing deposits in gas scrubbers which is effective over a wide range of pH conditions.
These and other objects of this invention are accomplished by the composition and method of this invention in which from about 0.01 to about 100 ppm active ingredients, preferably from about 0.1 to about 10 ppm active ingredients, of a composition comprising a scale inhibitor and a high molecular weight polymer is maintained in the aqueous scrubbing medium. The ratio of scale inhibitor to high molecular weight polymer may be from about 1:10 to about 10:1, preferably from about 1:5 to about 5:1, by weight.
Suitable threshold scale inhibitors include phosphonates of the general formula: ##STR1## wherein R is ##STR2## M is H, NH.sub.4, alkali metal or combinations thereof; n is 0 to 6; and
x is 1 to 6; and those of the general formula: ##STR3## wherein X is -- OH or -- NH.sub.2 and R is an alkyl group of from 1 to 5 carbon atoms. The most preferred compounds, however, are amino tris (methylene phosphonic acid) and hydroxyethylidene-1, 1-diphosphonic acid (HEDP) and water-soluble salts thereof.
Also useful as threshold scale inhibitors are phosphates such as polyphosphates, as for example sodium polyphosphates and phosphate esters of the formula: EQU N [ROPO.sub.3 M.sub.2 ].sub.3
wherein M may be -- H, -- NH.sub.2 or a monovalent metal ion and R is an alkylene group having from 1 to 18 carbon atoms. Suitable polyphosphates are disclosed in U.S. Pat. Nos. 2,337,856, 2,906,599 and 3,213,017.
Suitable high molecular weight polymers include any nonionic or anionic water-soluble polymer having a molecular weight of at least 100,000, preferably at least 250,000, and the most preferred polymers having a molecular weight of at least 1,000,000. Examples of suitable polymers include polyacrylamides such as those set forth in U.S. Pat. No. 3,085,916, polymers of 2-acrylamido methyl propane sulfonic acid such as those set forth in U.S. Pat. No. 3,709,816, and sulfonated polystyrenes such as those set forth in U.S. Pat. No. 3,630,937.
The invention is further illustrated by the following examples: